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Effects of humidity and temperature on a proton exchange membrane fuel cell (PEMFC) stack
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Journal of Industrial and Engineering Chemistry 14 (2008) 357–364 www.elsevier.com/locate/jiec
Effects of humidity and temperature on a proton exchange membrane fuel cell (PEMFC) stack Sunhoe Kim b, Inkwon Hong a,* b
a Department of Chemical Engineering, Dankook University, Gyunggi-do 448-701, South Korea Department of Minerals & Mining Engineering, Sangji University, Gangwon-do 220-702, South Korea
Received 27 September 2007; accepted 21 January 2008
Abstract Various operation conditions such as fuel and oxidant humidity and stack temperature were evaluated in order to improve the performance of proton exchange membrane fuel cell (PEMFC). Under the experimental environment, the resultant data were obtained with a stack of 10 cm2 active area and 10 cells. The humidity was strongly dependent on the temperature of anode and cathode as well as stack temperature change. Compared to anode humidity, the stack performance of PEMFC was more affected by cathode humidity and increased with increasing cathode humidity. In the viewpoint of stack performance, the stack temperature was less influenced than humidity. # 2008 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. Keywords: PEMFC; Fuel cell; Oxidant humidity; Stack temperature; Anode; Cathode
1. Introduction Fuel cells are regarded as one of a strong candidate as an alternative energy source for next generation. There are many different kinds of fuel cells, such as, solid oxide fuel cell (SOFC), molten carbon fuel cell (MCFC), direct methanol fuel cell (DMFC), and alkaline fuel cell (AFC). Among these various kinds of fuel cell proton exchange membrane fuel cell (PEMFC) is the strongest candidate for technological maturity for commercialization in near future in transportation and stationary applications field. A fuel cell stack consists of bipolar plates (BPs), membrane electrode assemblies (MEAs), gas diffusion layer (GDL), current collectors, end plates and sealants. These parts of stack govern the cost and performance of a fuel cell stack. The performance and durability of a fuel cell stack is also influenced by operating conditions and design of a fuel cell stack, such as, stacking compression, flow field geometry and manifold design. Lee et al. [1] explained the correlation between compression and gas diffusion in GDL in their experimental work. The fuel cell performance, durability and cost are also influenced by fuel cell parts material, such as membrane, catalyst, and bipolar plates. The modification of
* Corresponding author. Tel.: +82 31 8005 3544; fax: +82 31 8005 3536. E-mail address: [email protected] (I. Hong).
acid–base polymer membrane has been reported by Lee et al. [2]. They explained various kinds of membrane to be used as electrolyte of PEMFC in their review paper. They modified perfluorosulfonic acid membrane to increase thermal stabilities and higher proton conductivities by recasting with inorganic additives. Kim and Lim [3] also showed different kinds of membrane material, such as polybenzy imidazol (PBI) to increase the operation temperature up to 180 8C. The platinum catalyst has also been reported by Kwon et al. through experimental work [4]. They explained that increased platinum catalyst surface area would improve performance of a PEMFC. Developing those materials is demanded to commercialize fuel cell system for cost reduction and performance enhancement. However, the development of those materials demands patience in terms of time and cost. PEMFC stack design, such as, geometries of manifold and flow field, concerns with fuel and oxidant distribution, liquid water removal and pressure drop in a fuel cell stack along with channel length. Our previous work [5] elucidated the flow field design effect on fuel cell performance. It explained how pressure drop and contact resistance affect on the performance of a fuel cell. Kim et al. [6] showed the flow field effect on the performance of a fuel cell in their transient experimental system. They performed with two different types of flow fields, single and three-channel flow path, in their work. They showed the longer channel yielded better performance than that of the
1226-086X/$ – see front matter # 2008 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. doi:10.1016/j.jiec.2008.01.007
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shorter channel length in terms of both steady and transient condition. The gas distributions, manifolds, in a fuel cell stack have been explained in Refs. [7,8]. Karimi et al. [7] emphasized the importance of pressure loss ratio between manifold and bipolar plate. They compared the types of manifolds, Uconfiguration and Z-configuration types. From their numerical work the U-configuration manifold yields lower pressure drop than that of Z-configuration manifold. They also explained the flow channels with smaller cross-sectional area and longer lengths increase the pressure drop of flow field and gas distribution. Jiao et al. [8] explained the correlation of liquid water transport with manifold. They suggested manifold design in terms of liquid water removal, by keeping unit cell and MEA side of the gas flow close to the outlet of the outflow manifold and serpentine gas flow channel’s ‘‘collecting-and-separatingeffect’’ to facilitate water removal. They emphasized that the importance of water removal to get stable performance. The performance, stability, and durability of a PEMFC stack are strongly dependent on operation condition, such as humidity and temperature. Most of the operating conditions are concerned with water management. Water management is important for a PEMFC stack operation. The membrane should retain water inside of the membrane so that it can transfer protons to the other side. Inadequate liquid water content in membrane, drying, may cause resistance on ion transport in the membrane, which causes performance loss. On the other hand, too much water or flooding causes flooding in flow field and GDL, which inhibits mass transport onto the electrode surface. It also causes performance degradation, and further, permanent damage on cell. Those problems presented above are strongly related with both operating condition and design of the fuel cell stack. The effect of channel structure on fuel cell performance has been presented in our previous work [5]. The effect of humidity and temperature
on a fuel cell stack performance is explained in this paper. The water management effect in terms of humidification change was performed and reported in this paper. Therefore, the water management was limited as humidity condition change in this paper. Further studies for water contents in membrane and the correlation between conductivity and water contents will be conducted in our next work. Lee and co-workers [9] mentioned the water management in their experimental and numerical work. They tried to explain numerically how the cell performance affected with hydration in the membrane in terms of water distribution in the membrane. There are many experimental and numerical works regarding humidity control and temperature effect on fuel cell performance [10–12]. Saleh et al. [10] presented their experimental work with humidity and temperature control with a 25 cm2 single cell. Amirinejad et al. [11] also mentioned the operating parameters, humidity, temperature and pressure with their 5 cm2 single cell. They tried to observe and explain the operation parameter effect. Zhang et al. [12] tried to operate the cell wide range of temperature, 23–120 8C, and dry operation with their small single cell, 4.4 cm2. Numerical works for the importance of temperature and humidity had also been presented [13–15]. However, most of those papers have been presented with data of single cell. The operation with a fuel cell stack may be more realistic than that of a single cell in terms of temperature gradient. Also, small active area can hardly show temperature and fuel distribution. Using a PEMFC stack of 10 cells and 100 cm2 data has been obtained and presented in this work. 2. Experimental Various kinds of operation conditions concerning temperature and humidity have been presented in this work. A PEMFC
Fig. 1. Schematic of the experimental setup for fuel cell operation.
S. Kim, I. Hong / Journal of Industrial and Engineering Chemistry 14 (2008) 357–364
fuel cell stack of 10 cells and 100 cm2 of active are was used to observe stack temperature and humidity condition. The MEAs used in this work were the combination of Nafion 112 s with thickness of 50 mm and catalyst loading of 4.5 and 6.0 mg/cm2, Pt/Ru and Pt, anode and cathode, respectively. The GDLs were 10BC, a SGL Carbon product, with nominal thickness of
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400 mm. A model 890B (Scribner Associated) electric load was used to control electric load and collect data with a computer. Digital mass flow controllers and products of MKS were used to control the flow rates of fuel and oxidant. And mass flow controller was calibrated with a bubble flow meter. Two bubbler-typed humidifiers were used to control both anode and
Fig. 2. Polarization behavior and cell voltage distribution at Tstack = 70 8C under anode full humidification and various cathode humidity conditions, 40, 60, 80 and 100%. (a) Polarization behaviors for various cathode humidity conditions: 40% (^), 60% (&), 80% (~) and 100% (*). (b) Cell voltage distribution for 40% of cathode humidity. (c) Cell voltage distribution for 60% of cathode humidity. (d) Cell voltage distribution for 80% of cathode humidity. (e) Cell voltage distribution for 100% of cathode humidity.
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cathode humidifications. The humidifications for both anode fuel and cathode oxidant were controlled by changing the humidifier bottle temperature. The stack temperature was controlled with cooling water controlled by external heating/ cooling system. The stoichiometry of hydrogen and oxidant gas were 1.4 and 2.0, respectively, in this work. The fuel and oxidant gas were hydrogen, 99.999%, and industrial air,
respectively. The stack temperature was decided as outlet temperature of cooling water from the stack used in this paper. The schematic of the experimental setup is illustrated in Fig. 1. The hydrogen and air flow rates were controlled with mass flow controllers. The gases were humidified with humidifiers bottle. The dry gas enters into the humidifier and the gas contact as bubble in the humidifier. The gas humidity is controlled by
Fig. 3. Polarization behavior and cell voltage distribution at Tstack = 70 8C under dry anode and various cathode humidity conditions, 40, 60, 80 and 100%. (a) Polarization behaviors for various cathode humidity conditions: 40% (^), 60% (&), 80% (~) and 100% (*). (b) Cell voltage distribution for 40% of cathode humidity. (c) Cell voltage distribution for 60% of cathode humidity. (d) Cell voltage distribution for 80% of cathode humidity. (e) Cell voltage distribution for 100% of cathode humidity.
S. Kim, I. Hong / Journal of Industrial and Engineering Chemistry 14 (2008) 357–364
changing humidifier temperature. The steady state operation, polarization curves, were obtained by changing current and wait until the voltage converges to stable values at each step. The experiments were performed with various kinds of stack temperature and humidifications of anode and cathode. Two
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stack temperatures, 60 and 70 8C, were selected in this experimental work. The anode humidity was controlled in two different conditions, full humidification and dry operation. The cathode humidifications are controlled in several steps to observe fuel cell performance at each humidification condi-
Fig. 4. Polarization behavior and cell voltage distribution at Tstack = 60 8C under anode full humidification and various cathode humidity conditions, 40, 60, 80 and 100%. (a) Polarization behaviors for various cathode humidity conditions: 40% (^), 60% (&), 80% (~) and 100% (*). (b) Cell voltage distribution for 40% of cathode humidity. (c) Cell voltage distribution for 60% of cathode humidity. (d) Cell voltage distribution for 80% of cathode humidity. (e) Cell voltage distribution for 100% of cathode humidity.
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tions. The cathode humidity was controlled with cathode humidity bottle dew point temperature according to the stack temperature. The humidity was decided according to the stack temperature. For example, at the condition of 70 8C of gas
humidity bottle’s dew point and stack temperature of 70 8C, the relative humidity of the gas is 100%. The fuel cell performance is measured and recorded with those operating condition changes.
Fig. 5. Polarization behavior and cell voltage distribution at Tstack = 60 8C under dry anode and various cathode humidity conditions, 40, 60, 80 and 100%. (a) Polarization behaviors for various cathode humidity conditions: 40% (^), 60% (&), 80% (~) and 100% (*). (b) Cell voltage distribution for 40% of cathode humidity. (c) Cell voltage distribution for 60% of cathode humidity. (d) Cell voltage distribution for 80% of cathode humidity. (e) Cell voltage distribution for 100% of cathode humidity.
S. Kim, I. Hong / Journal of Industrial and Engineering Chemistry 14 (2008) 357–364
3. Results and discussions The experiments were illustrated with a PEMFC stack of 100 cm2 and 10 cells. Two different temperatures, dry and 100% of fuel humidification and various oxidant humidification conditions have been performed for this experimental work. The cathode humidification effect on 70 8C of stack temperature and full humidification of fuel is compared in Fig. 2(a). This figure shows the polarization behaviors for each cathode humidification condition. The stack performance at each cathode humidity condition was performed in this figure. The performances presented in this figure are in the form of mean cell voltage of each cell of the stack used in this work. The mean cell performances at 200 mA/cm2 are collected as 0.711, 0.722, 0.738 and 0.740 V for 40, 60, 80 and 100% of cathode relative humidity, respectively. This performance comparison shows the cathode humidification effect under 70 8C of stack temperature and full humidification of anode on PEMFC stack performance. The cell voltage distribution of the stack is listed in Fig. 2(b)–(e) for cathode humidity of 40, 60, 80 and 100%, respectively. The cell voltage distribution is more uniform at higher cathode humidity. The far end cell from gas entrance was numbered as 1 and the closest cell from gas entrance numbered as 10. The condition of fully humidified anode and cathode is the most desired operating condition for a PEMFC stack performance is the best comparing to other conditions. However, those operating conditions may have difficulties in parts availability for system configuration and high operating cost. The other humidification and temperature cases are presented in next figures. Fig. 3(a) shows the comparison of mean cell voltage with cathode humidity changes under stack temperature of 70 8C and dry fuel of operating condition. The relative humidity of cathode is the same as in previous figure. The mean cell voltage for each cathode humidity step was 0.675, 0.699, 0.727 and 0.736 V for the cathode humidity of 40, 60, 80 and 100%, respectively. The performance was slightly lower than that of anode full humidification cases. The performances for each case are compared in Table 1. The performance at lower cathode humidity degrades significantly than that of anode humidified condition. The mean cell voltages at 200 mA/cm2 and cathode humidity of 40%, the lowest performance, were 0.711 and 0.675 V for full humidification and dry anode, respectively. These behaviors can be explained in the manner of water management. The water content in the membrane for anode dry condition is lower than that of anode full humidification since the water in membrane diffuses to dry anode side. These result in Table 1 Mean cell voltage of stack for operating conditions at 200 mA/cm2 Tstack (8C)
Anode humidity Full humidification 40
70 60 a
a
0.711 0.713
60
a
0.722 0.721
Cathode humidity (%).
80
Dry condition a
0.738 0.732
100
a
0.740 0.741
40a
60 a
80 a
100a
0.675 0.678
0.699 0.697
0.727 0.720
0.736 0.734
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degradation of ion conductivity in membrane. Also, the water diffusion to anode side becomes fast so that it inhibits proton drag from anode to cathode. The cell voltage distribution is shown in Fig. 3(b)–(e) for the cathode humidity of 40, 60, 80 and 100%, respectively. Fig. 4(a) shows the comparison of mean cell voltage of the stack at the temperature of 60 8C and 100% fuel humidification conditions. The relative humidity of cathode was controlled with cathode humidity bottle temperature changes, 40, 60, 80 and 100%. The mean cell voltages for this condition is quite similar to those at the stack temperature of 70 8C and full anode humidity in Fig. 2(a) for each cathode humidity step. The mean cell voltages at 200 mA/cm2 for each cathode humidity step were 0.731, 0.721, 0.732 and 0.741 V for the cathode humidity of 40, 60, 80 and 100%, respectively. The mean cell voltages at 40%, for an example, of cathode humidity at anode full humidification under the stack temperature of 60 and 70 8C at 200 mA/cm2 were 0.713 and 0.711 V, respectively. This result shows that the stack temperature rarely influences on the fuel cell stack’s performance. Rather, the stack performance is influenced with cathode humidification through this work. The cell voltage distribution at 200 mA/cm2 for these conditions is shown in Fig. 4(b)–(e). The cell voltage distribution looks quite good distribution in this operating condition. Fig. 5(a) shows the comparison of the cell voltage of stack temperature of 60 8C and dry fuel operating condition. Like the figures above the cathode relative humidity were the same as in previous figure, which were is 40, 60, 80 and 100%. The fuel cell stack performances are also expressed as mean cell voltage in this figure. The performance of the fuel cell stack is similar to that of 70 8C stack temperature and dry anode humidity. This figure also shows that the fuel cell stack performance is rarely influenced by stack temperature changes. This figure also shows the performance of the fuel cell stack is mainly influenced by the gas humidity changes. The performance degrades significantly than that of Fig. 4(a), 60 8C of stack temperature and full humidification of anode. The mean cell voltages at 200 mA/cm2 under 40% of cathode humidity are 0.713 and 0.678 V for anode full humidification and dry, respectively. The stack performance under 40% of cathode humidity condition at the stack temperature of 70 and 60 8C were 0.675 and 0.678 V, respectively. This result shows that the stack performance is rarely influenced with stack temperature changes. Rather it is affected with the humidity of anode and cathode. 4. Conclusions Data are presented to show how the fuel cell performance influenced with temperature and humidification. The fuel cell performance is less influenced with temperature change. Rather, the anode and cathode humidification condition effects on fuel cell performance seriously. The fuel cell stack performance is affected seriously with cathode humidification changes. The comparison of fuel cell performance for full humidified and dried anode condition has presented in this experimental work. The anode humidification also affects fuel cell performances. The comparison of stack temperature, 70
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and 60 8C, shows less influence on fuel cell performance. This paper presents the initial performance of fuel cell stack’s performances. The long-term durability test will be requested for proper fuel cell effect. The fuel cell operating condition needs to be optimized in terms of operating cost, BOP integration and long-term durability. The fuel cell operation parameters for those cases were defined in this paper. This paper suggests the necessity of optimization for operation conditions including humidification and temperature.
[3] [4] [5] [6] [7] [8] [9] [10] [11]
References [12] [1] W-K. Lee, C-H. Ho, J.W. Van Zee, M. Murthy, J. Power Sources 84 (1999) 45. [2] J. Lee, N. Quan, J. Hwang, S. Lee, H. Kim, H. Lee, H. Kim, J. Ind. Eng. Chem. 12 (2) (2006) 175.
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H. Kim, T. Lim, J. Ind. Eng. Chem. 10 (7) (2004) 1081. O. Kwon, H. Ryu, S. Jung, J. Ind. Eng. Chem. 12 (2) (2006) 306. S. Kim, I.K. Hong, J. Ind. Eng. Chem. 13 (5) (2007) 864. S. Kim, S. Shimpalee, J.W. Van Zee, J. Electrochem. Soc. 152 (2005) A1265. G. Karimi, J.J. Baschuck, X. Li, J. Power Sources 147 (2005) 162. K. Jiao, B. Zhou, P. Quan, J. Power Sources 154 (2006) 124. S. Shimpalee, W-K. Lee, J.W. Van Zee, J. Electrochem. Soc. 150 (2003) A341. M.M. Saleh, T. Okajima, M. Hayase, F. Kitamura, T. Ohsaka, J. Power Sources 164 (2007) 503. M. Amirinejad, S. Rowshanzamir, M.H. Eikani, J. Power Sources 161 (2006) 872. J. Zhang, Y. Tang, C. Song, X. Cheng, J. Zhang, H. Wang, Electrochim. Acta 25 (2007) 5095. C. Lee, H.-S. Chu, J. Power Sources 161 (2006) 949. G.H. Guvelioglu, H.G. Stenger, J. Power Sources 163 (2007) 882. Y. Zong, B. Zhou, A. Sobiesiak, J. Power Sources 161 (2006) 143.
Journal of Industrial and Engineering Chemistry 14 (2008) 357–364 www.elsevier.com/locate/jiec
Effects of humidity and temperature on a proton exchange membrane fuel cell (PEMFC) stack Sunhoe Kim b, Inkwon Hong a,* b
a Department of Chemical Engineering, Dankook University, Gyunggi-do 448-701, South Korea Department of Minerals & Mining Engineering, Sangji University, Gangwon-do 220-702, South Korea
Received 27 September 2007; accepted 21 January 2008
Abstract Various operation conditions such as fuel and oxidant humidity and stack temperature were evaluated in order to improve the performance of proton exchange membrane fuel cell (PEMFC). Under the experimental environment, the resultant data were obtained with a stack of 10 cm2 active area and 10 cells. The humidity was strongly dependent on the temperature of anode and cathode as well as stack temperature change. Compared to anode humidity, the stack performance of PEMFC was more affected by cathode humidity and increased with increasing cathode humidity. In the viewpoint of stack performance, the stack temperature was less influenced than humidity. # 2008 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. Keywords: PEMFC; Fuel cell; Oxidant humidity; Stack temperature; Anode; Cathode
1. Introduction Fuel cells are regarded as one of a strong candidate as an alternative energy source for next generation. There are many different kinds of fuel cells, such as, solid oxide fuel cell (SOFC), molten carbon fuel cell (MCFC), direct methanol fuel cell (DMFC), and alkaline fuel cell (AFC). Among these various kinds of fuel cell proton exchange membrane fuel cell (PEMFC) is the strongest candidate for technological maturity for commercialization in near future in transportation and stationary applications field. A fuel cell stack consists of bipolar plates (BPs), membrane electrode assemblies (MEAs), gas diffusion layer (GDL), current collectors, end plates and sealants. These parts of stack govern the cost and performance of a fuel cell stack. The performance and durability of a fuel cell stack is also influenced by operating conditions and design of a fuel cell stack, such as, stacking compression, flow field geometry and manifold design. Lee et al. [1] explained the correlation between compression and gas diffusion in GDL in their experimental work. The fuel cell performance, durability and cost are also influenced by fuel cell parts material, such as membrane, catalyst, and bipolar plates. The modification of
* Corresponding author. Tel.: +82 31 8005 3544; fax: +82 31 8005 3536. E-mail address: [email protected] (I. Hong).
acid–base polymer membrane has been reported by Lee et al. [2]. They explained various kinds of membrane to be used as electrolyte of PEMFC in their review paper. They modified perfluorosulfonic acid membrane to increase thermal stabilities and higher proton conductivities by recasting with inorganic additives. Kim and Lim [3] also showed different kinds of membrane material, such as polybenzy imidazol (PBI) to increase the operation temperature up to 180 8C. The platinum catalyst has also been reported by Kwon et al. through experimental work [4]. They explained that increased platinum catalyst surface area would improve performance of a PEMFC. Developing those materials is demanded to commercialize fuel cell system for cost reduction and performance enhancement. However, the development of those materials demands patience in terms of time and cost. PEMFC stack design, such as, geometries of manifold and flow field, concerns with fuel and oxidant distribution, liquid water removal and pressure drop in a fuel cell stack along with channel length. Our previous work [5] elucidated the flow field design effect on fuel cell performance. It explained how pressure drop and contact resistance affect on the performance of a fuel cell. Kim et al. [6] showed the flow field effect on the performance of a fuel cell in their transient experimental system. They performed with two different types of flow fields, single and three-channel flow path, in their work. They showed the longer channel yielded better performance than that of the
1226-086X/$ – see front matter # 2008 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. doi:10.1016/j.jiec.2008.01.007
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shorter channel length in terms of both steady and transient condition. The gas distributions, manifolds, in a fuel cell stack have been explained in Refs. [7,8]. Karimi et al. [7] emphasized the importance of pressure loss ratio between manifold and bipolar plate. They compared the types of manifolds, Uconfiguration and Z-configuration types. From their numerical work the U-configuration manifold yields lower pressure drop than that of Z-configuration manifold. They also explained the flow channels with smaller cross-sectional area and longer lengths increase the pressure drop of flow field and gas distribution. Jiao et al. [8] explained the correlation of liquid water transport with manifold. They suggested manifold design in terms of liquid water removal, by keeping unit cell and MEA side of the gas flow close to the outlet of the outflow manifold and serpentine gas flow channel’s ‘‘collecting-and-separatingeffect’’ to facilitate water removal. They emphasized that the importance of water removal to get stable performance. The performance, stability, and durability of a PEMFC stack are strongly dependent on operation condition, such as humidity and temperature. Most of the operating conditions are concerned with water management. Water management is important for a PEMFC stack operation. The membrane should retain water inside of the membrane so that it can transfer protons to the other side. Inadequate liquid water content in membrane, drying, may cause resistance on ion transport in the membrane, which causes performance loss. On the other hand, too much water or flooding causes flooding in flow field and GDL, which inhibits mass transport onto the electrode surface. It also causes performance degradation, and further, permanent damage on cell. Those problems presented above are strongly related with both operating condition and design of the fuel cell stack. The effect of channel structure on fuel cell performance has been presented in our previous work [5]. The effect of humidity and temperature
on a fuel cell stack performance is explained in this paper. The water management effect in terms of humidification change was performed and reported in this paper. Therefore, the water management was limited as humidity condition change in this paper. Further studies for water contents in membrane and the correlation between conductivity and water contents will be conducted in our next work. Lee and co-workers [9] mentioned the water management in their experimental and numerical work. They tried to explain numerically how the cell performance affected with hydration in the membrane in terms of water distribution in the membrane. There are many experimental and numerical works regarding humidity control and temperature effect on fuel cell performance [10–12]. Saleh et al. [10] presented their experimental work with humidity and temperature control with a 25 cm2 single cell. Amirinejad et al. [11] also mentioned the operating parameters, humidity, temperature and pressure with their 5 cm2 single cell. They tried to observe and explain the operation parameter effect. Zhang et al. [12] tried to operate the cell wide range of temperature, 23–120 8C, and dry operation with their small single cell, 4.4 cm2. Numerical works for the importance of temperature and humidity had also been presented [13–15]. However, most of those papers have been presented with data of single cell. The operation with a fuel cell stack may be more realistic than that of a single cell in terms of temperature gradient. Also, small active area can hardly show temperature and fuel distribution. Using a PEMFC stack of 10 cells and 100 cm2 data has been obtained and presented in this work. 2. Experimental Various kinds of operation conditions concerning temperature and humidity have been presented in this work. A PEMFC
Fig. 1. Schematic of the experimental setup for fuel cell operation.
S. Kim, I. Hong / Journal of Industrial and Engineering Chemistry 14 (2008) 357–364
fuel cell stack of 10 cells and 100 cm2 of active are was used to observe stack temperature and humidity condition. The MEAs used in this work were the combination of Nafion 112 s with thickness of 50 mm and catalyst loading of 4.5 and 6.0 mg/cm2, Pt/Ru and Pt, anode and cathode, respectively. The GDLs were 10BC, a SGL Carbon product, with nominal thickness of
359
400 mm. A model 890B (Scribner Associated) electric load was used to control electric load and collect data with a computer. Digital mass flow controllers and products of MKS were used to control the flow rates of fuel and oxidant. And mass flow controller was calibrated with a bubble flow meter. Two bubbler-typed humidifiers were used to control both anode and
Fig. 2. Polarization behavior and cell voltage distribution at Tstack = 70 8C under anode full humidification and various cathode humidity conditions, 40, 60, 80 and 100%. (a) Polarization behaviors for various cathode humidity conditions: 40% (^), 60% (&), 80% (~) and 100% (*). (b) Cell voltage distribution for 40% of cathode humidity. (c) Cell voltage distribution for 60% of cathode humidity. (d) Cell voltage distribution for 80% of cathode humidity. (e) Cell voltage distribution for 100% of cathode humidity.
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cathode humidifications. The humidifications for both anode fuel and cathode oxidant were controlled by changing the humidifier bottle temperature. The stack temperature was controlled with cooling water controlled by external heating/ cooling system. The stoichiometry of hydrogen and oxidant gas were 1.4 and 2.0, respectively, in this work. The fuel and oxidant gas were hydrogen, 99.999%, and industrial air,
respectively. The stack temperature was decided as outlet temperature of cooling water from the stack used in this paper. The schematic of the experimental setup is illustrated in Fig. 1. The hydrogen and air flow rates were controlled with mass flow controllers. The gases were humidified with humidifiers bottle. The dry gas enters into the humidifier and the gas contact as bubble in the humidifier. The gas humidity is controlled by
Fig. 3. Polarization behavior and cell voltage distribution at Tstack = 70 8C under dry anode and various cathode humidity conditions, 40, 60, 80 and 100%. (a) Polarization behaviors for various cathode humidity conditions: 40% (^), 60% (&), 80% (~) and 100% (*). (b) Cell voltage distribution for 40% of cathode humidity. (c) Cell voltage distribution for 60% of cathode humidity. (d) Cell voltage distribution for 80% of cathode humidity. (e) Cell voltage distribution for 100% of cathode humidity.
S. Kim, I. Hong / Journal of Industrial and Engineering Chemistry 14 (2008) 357–364
changing humidifier temperature. The steady state operation, polarization curves, were obtained by changing current and wait until the voltage converges to stable values at each step. The experiments were performed with various kinds of stack temperature and humidifications of anode and cathode. Two
361
stack temperatures, 60 and 70 8C, were selected in this experimental work. The anode humidity was controlled in two different conditions, full humidification and dry operation. The cathode humidifications are controlled in several steps to observe fuel cell performance at each humidification condi-
Fig. 4. Polarization behavior and cell voltage distribution at Tstack = 60 8C under anode full humidification and various cathode humidity conditions, 40, 60, 80 and 100%. (a) Polarization behaviors for various cathode humidity conditions: 40% (^), 60% (&), 80% (~) and 100% (*). (b) Cell voltage distribution for 40% of cathode humidity. (c) Cell voltage distribution for 60% of cathode humidity. (d) Cell voltage distribution for 80% of cathode humidity. (e) Cell voltage distribution for 100% of cathode humidity.
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tions. The cathode humidity was controlled with cathode humidity bottle dew point temperature according to the stack temperature. The humidity was decided according to the stack temperature. For example, at the condition of 70 8C of gas
humidity bottle’s dew point and stack temperature of 70 8C, the relative humidity of the gas is 100%. The fuel cell performance is measured and recorded with those operating condition changes.
Fig. 5. Polarization behavior and cell voltage distribution at Tstack = 60 8C under dry anode and various cathode humidity conditions, 40, 60, 80 and 100%. (a) Polarization behaviors for various cathode humidity conditions: 40% (^), 60% (&), 80% (~) and 100% (*). (b) Cell voltage distribution for 40% of cathode humidity. (c) Cell voltage distribution for 60% of cathode humidity. (d) Cell voltage distribution for 80% of cathode humidity. (e) Cell voltage distribution for 100% of cathode humidity.
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3. Results and discussions The experiments were illustrated with a PEMFC stack of 100 cm2 and 10 cells. Two different temperatures, dry and 100% of fuel humidification and various oxidant humidification conditions have been performed for this experimental work. The cathode humidification effect on 70 8C of stack temperature and full humidification of fuel is compared in Fig. 2(a). This figure shows the polarization behaviors for each cathode humidification condition. The stack performance at each cathode humidity condition was performed in this figure. The performances presented in this figure are in the form of mean cell voltage of each cell of the stack used in this work. The mean cell performances at 200 mA/cm2 are collected as 0.711, 0.722, 0.738 and 0.740 V for 40, 60, 80 and 100% of cathode relative humidity, respectively. This performance comparison shows the cathode humidification effect under 70 8C of stack temperature and full humidification of anode on PEMFC stack performance. The cell voltage distribution of the stack is listed in Fig. 2(b)–(e) for cathode humidity of 40, 60, 80 and 100%, respectively. The cell voltage distribution is more uniform at higher cathode humidity. The far end cell from gas entrance was numbered as 1 and the closest cell from gas entrance numbered as 10. The condition of fully humidified anode and cathode is the most desired operating condition for a PEMFC stack performance is the best comparing to other conditions. However, those operating conditions may have difficulties in parts availability for system configuration and high operating cost. The other humidification and temperature cases are presented in next figures. Fig. 3(a) shows the comparison of mean cell voltage with cathode humidity changes under stack temperature of 70 8C and dry fuel of operating condition. The relative humidity of cathode is the same as in previous figure. The mean cell voltage for each cathode humidity step was 0.675, 0.699, 0.727 and 0.736 V for the cathode humidity of 40, 60, 80 and 100%, respectively. The performance was slightly lower than that of anode full humidification cases. The performances for each case are compared in Table 1. The performance at lower cathode humidity degrades significantly than that of anode humidified condition. The mean cell voltages at 200 mA/cm2 and cathode humidity of 40%, the lowest performance, were 0.711 and 0.675 V for full humidification and dry anode, respectively. These behaviors can be explained in the manner of water management. The water content in the membrane for anode dry condition is lower than that of anode full humidification since the water in membrane diffuses to dry anode side. These result in Table 1 Mean cell voltage of stack for operating conditions at 200 mA/cm2 Tstack (8C)
Anode humidity Full humidification 40
70 60 a
a
0.711 0.713
60
a
0.722 0.721
Cathode humidity (%).
80
Dry condition a
0.738 0.732
100
a
0.740 0.741
40a
60 a
80 a
100a
0.675 0.678
0.699 0.697
0.727 0.720
0.736 0.734
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degradation of ion conductivity in membrane. Also, the water diffusion to anode side becomes fast so that it inhibits proton drag from anode to cathode. The cell voltage distribution is shown in Fig. 3(b)–(e) for the cathode humidity of 40, 60, 80 and 100%, respectively. Fig. 4(a) shows the comparison of mean cell voltage of the stack at the temperature of 60 8C and 100% fuel humidification conditions. The relative humidity of cathode was controlled with cathode humidity bottle temperature changes, 40, 60, 80 and 100%. The mean cell voltages for this condition is quite similar to those at the stack temperature of 70 8C and full anode humidity in Fig. 2(a) for each cathode humidity step. The mean cell voltages at 200 mA/cm2 for each cathode humidity step were 0.731, 0.721, 0.732 and 0.741 V for the cathode humidity of 40, 60, 80 and 100%, respectively. The mean cell voltages at 40%, for an example, of cathode humidity at anode full humidification under the stack temperature of 60 and 70 8C at 200 mA/cm2 were 0.713 and 0.711 V, respectively. This result shows that the stack temperature rarely influences on the fuel cell stack’s performance. Rather, the stack performance is influenced with cathode humidification through this work. The cell voltage distribution at 200 mA/cm2 for these conditions is shown in Fig. 4(b)–(e). The cell voltage distribution looks quite good distribution in this operating condition. Fig. 5(a) shows the comparison of the cell voltage of stack temperature of 60 8C and dry fuel operating condition. Like the figures above the cathode relative humidity were the same as in previous figure, which were is 40, 60, 80 and 100%. The fuel cell stack performances are also expressed as mean cell voltage in this figure. The performance of the fuel cell stack is similar to that of 70 8C stack temperature and dry anode humidity. This figure also shows that the fuel cell stack performance is rarely influenced by stack temperature changes. This figure also shows the performance of the fuel cell stack is mainly influenced by the gas humidity changes. The performance degrades significantly than that of Fig. 4(a), 60 8C of stack temperature and full humidification of anode. The mean cell voltages at 200 mA/cm2 under 40% of cathode humidity are 0.713 and 0.678 V for anode full humidification and dry, respectively. The stack performance under 40% of cathode humidity condition at the stack temperature of 70 and 60 8C were 0.675 and 0.678 V, respectively. This result shows that the stack performance is rarely influenced with stack temperature changes. Rather it is affected with the humidity of anode and cathode. 4. Conclusions Data are presented to show how the fuel cell performance influenced with temperature and humidification. The fuel cell performance is less influenced with temperature change. Rather, the anode and cathode humidification condition effects on fuel cell performance seriously. The fuel cell stack performance is affected seriously with cathode humidification changes. The comparison of fuel cell performance for full humidified and dried anode condition has presented in this experimental work. The anode humidification also affects fuel cell performances. The comparison of stack temperature, 70
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and 60 8C, shows less influence on fuel cell performance. This paper presents the initial performance of fuel cell stack’s performances. The long-term durability test will be requested for proper fuel cell effect. The fuel cell operating condition needs to be optimized in terms of operating cost, BOP integration and long-term durability. The fuel cell operation parameters for those cases were defined in this paper. This paper suggests the necessity of optimization for operation conditions including humidification and temperature.
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